Aircraft-borne optical sensors must be isolated from the environment to function properly and to preserve the aerodynamic efficiency of the aircraft design. This isolation is usually accomplished by placing the sensor behind a window. The window must be designed to provide the proper field of regard for the sensor. That is meant by the field of regard is the complete set of ordered pairs of values representing azimuth and elevation viewing angles through which the sensor can be pointed or gimbaled. In context of this specification, this is in contradistinction to the field of view which corresponds to the locus of points which the sensor can instantaneously observe given a particular orientation within the field of regard. Typically, the field of view of an optical sensor is significantly smaller than its field of regard.
Aircraft window design for isolating optical sensors is driven by two considerations: maintaining the aerodynamic efficiency of the overall aircraft design and the need to render the field of regard of the sensor as free of optical distortion and aberration as is practicable.
Conformal windows, which are windows having contours matching those of the surrounding surface of the aircraft in the context of the present application, offer the most favorable aerodynamic shape for maintaining the overall efficiency of the aircraft design. However, conformal windows create considerable optical aberration which varies greatly as the sensor is gimbaled or pointed through the field of regard.
As a consequence of the considerable optical aberration created by conformal windows and the difficulty of correcting such widely varying amounts of aberration, designers of on-board sensor systems typically prefer to use flat or spherical windows to protect sensors from the airborne environment. Although the use of non-conformal windows benefits the sensor designer, the aircraft suffers from increased resistance as a result of this design choice.
The principal types of optical aberration associated with conformal windows are coma and astigmatism. What is meant by coma in the context of the present application is the variation of magnification as a function of the aperture. Also in the context of the present application, astigmatism is the difference in focus location for fans of rays in the sagittal and tangential planes.
In previously known systems, the problem of correcting coma and astigmatism due to the use of a conformal window has not been resolved by optical means. A device for generating optical aberration has been previously disclosed, e.g. see Aberration Generator by R. A. Buchroeder and R. Brian Hooker, Journal of Applied Optics (1975), however this device provides limited amounts of optical aberration when compared to the amounts of optical aberration required in the context of the present invention. For this reason, the aberration generator disclosed in the above-mentioned reference is inadequate for the present application. As above mentioned, absent an acceptable optical solution to aberration-correction, designers are forced to use flat or spherical windows as above mentioned. Because these window designs reduce aerodynamic efficiency, smaller windows may be used to limit aerodynamic degradation. As a result, the sensor field of regard is limited.
What is needed is a system which provides an optical solution to the problem of correcting the optical aberration which results from the use of conformal windows in the isolation of aircraft-borne sensor systems, and, thus, allow for increased choices in aircraft window design. The system should be simple, easy to install and cost effective. The present system addresses such a need.